The present invention relates to a sealing system for steam turbines and particularly relates to a pipe or snout for flowing steam wherein the pipe is sealed to separate casings which have different magnitudes of thermal expansion relative to one another and hence are movable relative to one another and to the pipe.
Steam turbines require sealing systems that can prevent leakage between the steam in let snout and the surrounding distinct inner and outer shells and a nozzle box (hereafter sometimes collectively referred to as a housing). In current seal designs for this purpose, the seal system consists of sets of rings that seal between the inlet snout and each of the inner and outer shells and nozzle box. For example, for sealing in the annular s pac e between the snout and a shell, a plurality of sealing rings are axially stacked one against the other. Alternate sealing rings in the stack have large and small diameters, respectively. The smaller diameter sealing rings bear and seal against the exterior surface of the pipe or snout, while the larger diameter sealing rings bear and seal against the interior wall surface of the shell. Thus, with the rings alternately sealing radially against the snout and shell walls and sealing axially against one another at opposed axial sealing faces, relative movement between the parts is facilitated.
In one such prior sealing system, the smaller diameter sealing rings have a coefficient of thermal expansion less than the coefficient of thermal expansion of the snout whereby the snout expands a greater amount than the smaller sealing rings t o ensure a tight seal between the smaller diameter sealing rings and the snout wall as operating temperatures increase to steady state. In that same prior sealing system, the coefficient of thermal expansion of the larger diameter sealing rings is larger than the coefficient of thermal expansion of the outer shell such that the larger diameter rings expand more than the shell expands. This ensures a tight seal between the larger diameter sealing rings and the shell wall when the system heats up to operating temperature.
In these prior systems, however, there remain leakage paths due to the relative movement of the various parts of the system, e.g., misalignments and vibrations occur even at operating temperatures. Consequently, the sealing rings may lose contact with one another and/or the interfacing sealing component and yield significant leakage flow. With axially stacked sealing rings, the leakage flows may occur between the sealing rings and the snout or shell walls, or both, or between the axial sealing faces of the sealing rings per se. Accordingly, there is a need for a low leakage snout sealing system for steam turbines.
In accordance with a preferred embodiment of the present invention, there is provided a hybrid sealing assembly for further reducing leakage flow in a snout sealing system. The system includes axially stacked sealing rings for engaging the snout and shell walls and one or more additional secondary sealing elements. In one form hereof, secondary sealing elements extend in the radial plane of each sealing ring and seals between the sealing ring and one of the snout wall and the shell wall, i.e., the housing. In another form hereof, a sealing element extends axially between axially spaced sealing rings and engages both the snout wall and housing. The secondary sealing element(s), while redundant to the- sealing rings, reinforce the leakage performance of the sealing system. The secondary sealing elements choke the leakage flow that escapes past the sealing rings to minimize or preclude leakage flow.
In a first preferred embodiment hereof, each secondary seal comprises a frustoconical, generally C-shaped, annular sealing element disposed between a sealing ring and an adjoining wall, i.e., either the snout wall or the shell wall. Each sealing element lies in the radial plane of the sealing ring against which it lies in sealing engagement. Thus, the sealing surface of the C-shaped element at its opposite ends engages the radially adjacent sealing ring and a snout or housing wall. In another preferred embodiment hereof, the frustoconical, generally C-shaped, annular sealing element is enlarged and extends axially between axially spaced sealing rings and radially between the walls of the snout and housing. In this form, the primary sealing surfaces are between the C-shaped element and walls of the snout and housing. Tertiary sealing also may occur from axially applied fluid, e.g., steam pressure on the frustoconical sealing element causing the sealing element to engage the axially adjacent sealing rings.
In a further form of the present invention, piston rings are disposed alternately between the radial sealing elements and the radially spaced wall. For example, a piston ring is disposed between the shell or nozzle box wall and a sealing ring engageable with the snout wall. At an adjacent axial position, a piston ring is disposed between the sealing ring and the snout wall.
In a still further preferred embodiment of the present invention, a plurality of piston ring carriers are axially stacked in the annular seal cavity between the snout and the shell or nozzle box and axially spaced from a plurality of sealing rings. Each piston ring carrier has a radial cavity for holding a piston ring. For example, the piston ring carrier at one end of the axial stack of sealing elements has a radially inwardly opening cavity for receiving a piston ring or rings for bearing against the snout wall. The next axially spaced piston ring carrier has a radially outwardly opening cavity for receiving a piston ring or rings engageable against the shell wall. The piston ring carriers seal axially one against the other with the piston rings sealing between the carriers and the walls of the snout and shell, respectively. The remaining sealing rings in the stack serve as secondary seals as previously described.
In a preferred embodiment according to the present invention, there is provided a low leakage sealing system for a turbine, comprising a pipe for flowing a heated fluid medium, a housing surrounding the pipe and spaced radially outwardly of the pipe defining an annular space therewith, a plurality of large and small-diameter sealing rings alternately disposed in the annular space about the axis for engagement with respective surfaces of the housing and the pipe to form seals therewith, the sealing rings engaging one another in an axial direction to form seals along adjoining axial faces thereof and at least one element disposed between one of the sealing rings and one of the pipe and the housing for choking leakage flow past the sealing ring.
In a further preferred embodiment according to the present invention, there is provided a low leakage sealing system for a turbine, comprising a pipe for flowing a heated fluid medium, a housing surrounding the pipe and spaced radially outwardly of the pipe defining an annular space therewith and first and second carrier rings bearing against one another in the annular space and about the axis, the first and second carrier rings having radially inwardly and outwardly opening cavities, respectively, a sealing ring disposed in each cavity and bearing in sealing relation against the pipe and the housing, respectively, the carrier rings having axially adjoining sealing surfaces.